Many people are happy to share their photos on the internet just as I am. Its nice to get oooh's and ahhh's and sometimes the constructive criticism but its not nice to learn one of your images was stolen and used without your consent, or at least its not something I enjoy. I have met people though that could care less if someone were to steal their images without any credit in return but one would think that a majority of people who spend lots of time and money on their creative hobbies like to be compensated or at least credited for their efforts if someone were to use their work.
So, how does one protect the images they post online? Well, there are some ways to deter image theft but really, one should consider any image posted on the internet to be open to theft. If you have an image that so dear and important to you that you don't want anyone else to claim it or use it... then best advice is not to post it on the net. It kinda follows the saying, "if there's a will, there's a way" meaning that if someone really wants your image that was posted on the net... they will find a way to get it.
Using low resolution images to deter theft. This is a help to deter image theft but definitely not the best method of protection. Consider today's average monitor resolution to be 1024x768, its silly to post pictures on the net larger than this unless you need someone to critique an image for its detail. An image that is larger is wasted file size and more useful as a printed piece to anyone that wanted to steal it. Consider that a 1024px by 678px image is nearly equal to a 4x6" 300DPI print and actually most people could print a nice 4x6 or possibly 5x7 and would be happy with the quality because many people aren't overly concerned with subtle details and only care about the image as a whole.
I know, some of you are thinking, "but I made my images 72DPI or 72PPI so they would be horrible for a print and should be safe". That idea is somewhat true but... a resolution of 72 for a large picture just means that large picture won't print great at the larger sizes.... reduce the physical size of the image and suddenly that 72DPI image can look as good as the 300DPI version when printed if scaled down enough.
Consider this... on a daily basis, I get art to use for print advertising that came from the internet. So its almost always 72DPI. How do I make use of this art? Normally, I open the art in Photoshop, reduce the physical dimensions by 50% and then re-sample the image to 200 DPI which is our printing resolution. With this formula, I am enlarging the actual image slightly as I meet the 200DPI resolution because technically, I should have gone to 144DPI since I only reduce physical size by 50%. I have found that Photoshop does well enough in enlarging the image however that this fractional amount barely degrades the image and I get a larger image this way. Enlarging the image anymore than this however will show degradation in quality as the image becomes larger.
Bottom line, if you really want to protect your images, then either do not post them to the internet or, post them as small images 300px wide maybe or rely on watermarks as a deterrent. I personally post all my public display images no larger than 600px wide and I include my copyright tag on the bottom left corner. Someone could still grab this image and crop my copyright off but, the image would serve only useful on the screen really because a quality print from it would be pretty small such as 2 inches wide at 300DPI. So remember, consider the total width of pixels vs. DPI of a quality print when deciding how large to post your public treasures.